Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $21.27   3.773%   $22.0725   $22.07   $22.05   $22.10  
  Purchase #2 $12.35   5.317%   $13.0066   $13.01   $13.00   $13.00  
  Purchase #3 $186.62   4.423%   $194.8742   $194.87   $194.85   $194.90  
  Purchase #4 $88.54   4.630%   $92.6394   $92.64   $92.65   $92.60  
  Purchase #5 $11.26   13.170%   $12.7429   $12.74   $12.75   $12.70  
  Purchase #6 $5.64   6.632%   $6.014   $6.01   $6.00   $6.00  
  Purchase #7 $168.48   4.738%   $176.4626   $176.46   $176.45   $176.50  
  7 purchase totals = $494.16       $517.8122   $517.80   $517.75   $517.80